A novel method for image encryption by employing the diffraction imaging technique is proposed. This method is in principle suitable for most of the diffractive-imaging-based optical encryption schemes, and the classical double random phase encoding in the Fresnel domain is taken for an example for illustrating it. The encryption process is rather simple because only a single diffraction intensity pattern is needed to be recorded. The security keys include, in addition to the two phase only masks in the input and the Fresnel plane, the sparse data of the plaintext that can be obtained with a simple digital means. The sparse data serve as partial input plane support constraint in a phase retrieval algorithm, which is employed for completely retrieving the plaintext. Simulation results are presented to verify the validity of the proposed approach.